Assuming you are in CS4-
Window>Animations to open your animation palette
Make sure the "Animation" tab is open, not the "measurement" tab.
Hit the new piece of paper looking button in the animations palette to duplicate your current frame. This creates a copy of your "canvas" for the next frame. From here, you can change your layers and everything without affecting your previous frames.

Play around with it, its probably the easiest way to animate things, granted, the results can look pretty terrible if you are trying to keep it under 50KB for banners/